The invention relates to a method of manufacturing potassium-lithium-niobate crystals having a composition which corresponds to the formula EQU (K.sub.2 O).sub.0.3 (Li.sub.2 O).sub.0.2+a (Nb.sub.2 O.sub.5).sub.0.5+b
where
-0.01&lt;a&lt;0.01 PA0 -0.005&lt;b&lt;0.005
from a melt comprising potassium, lithium and niobium compounds. The invention also relates to crystals of this composition.
The above-mentioned monocrystalline crystals can be used in devices for doubling the frequency of a light wave. In these devices, this so-called primary wave is passed through a nonlinear optical medium of potassium-lithium-niobate (PLN), thereby forming a so-called second harmonic wave. The frequency of the latter wave is twice that of the primary wave.
The method described in the opening paragraph is known from, inter alia, European Patent Application EP 409.339. The method described therein enables stoichiometric potassium-lithium-niobate (PLN) having a non-centrosymmetrical tetragonal crystal structure of the tungsten bronze type to be manufactured. Using this material, the frequency of primary waves having a wavelength in the frequency range 790-920 nm can be doubled at room temperature. The known material also exhibits a high degree of birefringence in this frequency range, so that optimum phase matching of the (infra)red light (primary wave) and blue light (second harmonic wave) is possible. As the wavelength of the radiation issuing from the current semiconductor lasers, such as (Al,Ga)As lasers is situated in the range around 800 nm, the known material is very suitable for use in combination with such lasers. The generated shortwave light can be used for the high-density storage of information and for reading said information, for example in high-density data recording.
Applicants have found that the disadvantage of the known method is that the crystals manufactured are relatively small. For example, up to now it has been impossible to grow crystals having a crystal volume in excess of 3.5 mm.sup.3. This crystal volume is insufficient to allow said crystals to be successfully used in devices for frequency doubling. Besides, these relatively small crystals exhibited cracks and their composition was insufficiently homogeneous.